I hit my baby girl for the first time...

I feel like a horrible mother, an complete asshole.. I'll fill you in on the story.. I told my daughter (Alice, age 13) that I was going to be coming home late this afternoon (I'm a single parents) around 9 pm..
I trusted her with all my heart, to come home straight after school and do whatever she pleases at home, e.g. Watch t.v or whatever. 
My boss said that I could come home Early than expected (7pm) I was racing home and I tried to call her to see what she wanted from McDonald's. She didn't answer.. 
To my surprise I opened the front door to my house, seeing my daughter POUNCING on a boys penis!! (She told me it was her boyfriend!! Which I had no idea she had one)
I picked her up and slapped her so hard and threw the boy out of my house and called his parents.. I just can't believe what happened.. She was my baby girl, I never knew anything.. I feel terrible..